Development of a 9-months pregnant hybrid phantom and its internaldosimetry for thyroid agents

E. HOSEINIAN-AZGHADI, L. RAFAT-MOTAVALLI and H. MIRI-HAKIMABAD*

Physics Department, School of Sciences, Ferdowsi University of Mashhad, Azadi Square, Mashhad, 91775-1436, Iran*Corresponding author. Tel/Fax: +98-511-879-6983; Email: mirihakim@ferdowsi.um.ac.ir

(Received 1 August 2013; revised 5 December 2013; accepted 17 December 2013)

As a consequence of fetal radiosensitivity, the estimation of internal dose received by a fetus from radiophar-maceuticals applied to the mother is often important in nuclear medicine. A new 9-months pregnant phantombased on magnetic resonance (MR) images tied to the International Commission on Radiological Protection(ICRP) reference voxel phantom has been developed. Maternal and fetal organs were segmented from a set ofpelvic MR images of a 9-months pregnant subject using 3D-DOCTORTM and then imported into the 3D mod-eling software package RhinocerosTM for combining with the adult female ICRP voxel phantom and furthermodeling. Next, the phantom organs were rescaled to match with reference masses described in ICRPPublications. The internal anatomy of previous pregnant phantom models had been limited to the fetal brainand skeleton only, but the fetus model developed in this study incorporates 20 different organs. The currentreference phantom has been developed for application in comprehensive dosimetric study in nuclear medicine.The internal dosimetry calculations were performed for thyroid agents using the Monte Carlo transportmethod. Biokinetic data for these radiopharmaceuticals were used to estimate cumulated activity during preg-nancy and maternal and fetal organ doses at seven different maximum thyroid uptake levels. Calculating thedose distribution was also presented in a sagittal view of the pregnant model utilizing the mesh tally function.The comparisons showed, in general, an overestimation of the absorbed dose to the fetus and an underestima-tion of the fetal thyroid dose in previous studies compared with the values based on the current hybridphantom.

Keywords: internal dosimetry; thyroid agents; hybrid phantom; pregnancy; reference phantom

INTRODUCTION

Protection of a developing fetus against ionizing radiation isof particular interest. Estimations of the absorbed radiationdose to a fetus from a nuclear medicine procedure performedon the mother are an important component of stochastic riskassessment. The pregnant patient or worker has a right toknow the magnitude and type of potential radiation effectsthat might result from in utero exposure. If fetal dosesare above 1 mGy, a more detailed explanation should begiven [1].Stylized versions of pregnant phantoms were previously

developed by Stabin et al. [2], and subsequent organ refine-ments made by Chen [3], where fetal anatomy was modeledas an outer cylindrical shell of fetal skeleton with hemispher-ical ends, with an inner volume filled with generic fetal softtissue. Although used widely for reporting doses in diagnostic

nuclear medicine [4], this simplified treatment of fetalanatomy does not permit detailed assessment of the dose tomany fetal skeletal and tissue structures [5]. On the otherhand, in the 2007 Recommendations of the ICRP [6] it issuggested that doses from external and internal sourcesshould be calculated using reference computational phan-toms of the human body based on medical tomographicimages, replacing the use of various mathematical models.An image-based specimen-specific model of a 30-weeks

pregnant woman, presented by Shi and Xu [7], was con-structed from segmented computed tomography (CT)images. In 2008, Angel et al. published a 24-patient retro-spective study in which voxelized models of female abdom-inal anatomy were created covering a range of gestationalages of 5–36 weeks [8]. These voxelized models wererestricted to the region of the abdomenn and pelvis of apregnant woman.

Journal of Radiation Research, 2014, 55, 730–747doi: 10.1093/jrr/rrt223 Advance Access Publication 9 February 2014

© The Author 2014. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/ .0/), whichpermits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

4

Then Xu et al. [9] published the first fully hybrid series ofpregnant female models at the gestational ages of 12, 24 and36 weeks. The fetal internal anatomy of these models wasstill limited to the fetal brain and skeletal model. No furtherorgans of fetal anatomy had been incorporated into thesepregnant female phantoms.Recently, hybrid versions of human fetus were developed

by Maynard et al. for the fetal ages of 8, 10, 15, 20, 25, 30,35 and 38 weeks post-conception [5]. The Maynard et al.series of fetal hybrid computational phantoms contain fetalorgans, bone-specific details at various ages and weight per-centiles, however the fetus models were not located within apregnant female model.For reporting internal doses for image-based models of

pregnant woman as Russell et al. [4] did for mathematicalmodels, we decided to construct a series of hybrid computa-tional phantoms of pregnant females that included fetalorgans and real maternal internal anatomy according to mag-netic resonance (MR) images. In this paper we present thefirst member of our series: a model of a 9-months pregnantphantom together with estimations of internal dosimetry forvalidating the phantom. The current reference version canprovide internal dosimetric data applicable to nuclearmedicine.

MATERIALS ANDMETHODS

Phantom constructionMR image setsA set of MR images of a 9-months pregnant patient wasobtained from Picture Archive Communication System(PACS) at Qaem Hospital, Mashhad, Iran. The pregnantsubject was imaged using clinical MR. The soft tissue of the38-weeks’ fetus was sufficiently visualized under MR scan(performed on a Siemens Magnetom MRI system with 1.5 Tstatic field strength), and an acceptable image quality for ana-tomical modeling was obtained. Independent sagittal, axialand coronal T2 weighted scans were performed with para-meters as follows: image matrix of 256 × 256; number of 40,40 and 35 planes for sagittal, axial and coronal scans, re-spectively; slice thickness of 6 mm; slice gap of 1.2 mm;

pixel width of 1.5 mm for axial and sagittal scans; and 1.6 mmfor coronal scan.

Segmentation of MR image setsSegmentation of the MR image sets of the pregnant patientwas performed using 3D-DOCTORTM (Able SoftwareCorp., Lexington, MA), a 3D modeling and image-processing software package. The sagittal, axial and coronalimages were imported into 3D-DOCTOR, and the anatomic-al structures of interest were contoured manually using acomputer mouse (Fig. 1). Once all the necessary contourshad been completed, the export boundary function was usedto export the 3D contours as a Drawing Exchange file (*.dxf).3D-DOCTOR’s complex surface rendering functions also pro-vided a polygon mesh model for each image set. Eachpolygon mesh model was exported as a Wavefront Object file,a format that is easily imported into most 3D modeling soft-ware packages. (For more details please see Appendix A.)

NURBS and polygon mesh modeling and mergingto ICRP reference phantomThe female base phantom used in development of thepregnant hybrid phantom was a BREP version of the ICRPreference voxel phantom. Following the completion of seg-mentation, the polygon mesh ‘Wavefront Object’ files of theICRP female phantom and MR image sets were importedinto ‘RhinocerosTM’ (McNeel North America, Seattle, WA),a NURBS and polygon mesh modeling software package.Organs of the abdominal and pelvic region of the basephantom were replaced with those of the MR image sets.The polygon mesh models of maternal small intestine, spine,pelvis and fetal internal organs were left unaltered, whileother organs were converted to NURBS surfaces. These sur-faces are a powerful modeling tool that allows precise de-formation of individual volumes, which is a useful feature,particularly in regards to computational phantom construc-tion [5]. In constructing the pregnant phantom, ‘Rhinoceros’was used to (i) correctly orient the polygon mesh models,and (ii) incorporate NURBS surfaces into the phantom forvarious modeling requirements, including repair of segmen-tation artifacts, imparting deformability to certain structures,

Fig. 1. Segmentation of sagittal, coronal and axial MR image sets using 3D-Doctor software.

Development of a 9-months pregnant hybrid phantom 731

and adding skeletal bones and soft tissues that were not ini-tially segmented. NURBS and polygon mesh modeling ofthe pregnant phantom are presented in Fig. 2a and b.Replaced NURBS and polygon mesh models of maternaland fetal organs were then volumetrically rescaled to targetvolumes based on ICRP Publications 110 and 89. Carefulmanipulations of NURBS control points permit minimizingof the overlapped structures within the current version.(Further details of the NURBS and polygon mesh modelingmethodology are provided in Appendix B.)

VoxelizationThe voxelization process of the hybrid pregnant phantomwas done using an innovative method developed by our re-search group. Rhinoceros simple functions were used in thismethod to voxelize each organ. A network of lines wasdrawn with their intersections located in the center of thevoxels. Figure 3a shows the object to be voxelized later fromthe top viewport. The Rhinoceros ‘contour’ tool was used toextract the contours of the object (Fig. 3b). All the createdcontours were perpendicular to the z-axis, and the distancebetween them was equal to the voxel z-dimension. TheRhinoceros ‘trim’ tool was then applied to delete the lines’network outside the contours (Fig. 3c and d). So, the remain-ing lines of the network intersect at the center of the voxelsthat are located inside the object. The points could be easily

Fig. 2. (a) NURBS and polygon mesh modeling of the pregnantfemale and her fetus combined with the female ICRP referencevoxel phantom. (b) Female base phantom in comparison withcurrent model.

Fig. 3. (a) Drawing network of lines intersecting at the center of voxels and visual inspection of Object (stomach) forvoxelization process. (b) Contouring the object by using the Rhinoceros ‘contour’ tool. (c, d) Selecting the contours as a cuttingobject in the trimming process and trimming the lines. (e) Intersecting the remaining lines and exporting points.

E. Hoseinian-Azghadi et al.732

extracted using the Rhinoceros ‘intersect’ tool (Fig. 3e) andthen exported as a text file.This voxelization method is efficient because one only has

to draw the network of lines once. Another advantage of thismethod is visual inspection and manipulation of voxelsduring the voxelization process. Some tissues such as skin,walls of stomach, small intestine, large intestine, gallbladder, urinary bladder and the cortical region of the pelvisand spine were added to the phantom after voxelization byusing a FORTRAN program. For example, four layers ofvoxels at the surface of stomach were assigned as stomachwall. It is important to note that volumes obtained after therescaling process were calculated directly from RhinocerosTM;these values may not be identical to the volumes calculatedafter voxelization. Thus, a FORTRAN program was appliedto adjust reference volumes after voxelization by adding orremoving a few voxels to the exterior layers of organs.

Calculation of cumulated activitiesThe cumulated activities from two references were used inthis study to estimate organ doses for the current model [4,10]. Russell et al. [4] reported these values as residence timesof radiotracer in source organs during pregnancy, and consid-ered the fetus and/or the placenta in source regions. But, thebiokinetic data published in ICRP 53 [10] are assumed torepresent the average normal adult, and thus the non-pregnant woman. Generally, the assumption was made thatno changes occurred in the biodistribution of the radiophar-maceutical in maternal organs during pregnancy [4]. So, inorder to evaluate organ doses, cumulated activities assignedto the fetus and/or placenta were applied from Russell et al.’spublication.In addition, Russell et al. [11] provided a biokinetic model

of iodine during pregnancy. This compartmental model wasused to estimate cumulated activities of 131iodine and 123iodineby the method introduced in MIRD pamphlet No. 12 [12].A maple program was developed to solve the differentialequations of the compartmental model. The cumulated activ-ities of urinary bladder contents and the kidneys wereobtained by using the method published in ICRP 53 [10].

Estimation of organ doses and Monte CarlocalculationsIn this study, the organ doses were calculated from theMonte Carlo simulations, which were carried out separatelyfor photons and electrons. A general-purpose Monte Carlocode, MCNPX version 2.4.0, was employed to calculate theabsorbed dose for the pregnant model [13]. The phantomwas incorporated into the MCNPX lattice file. Organ- andtissue-specific densities and elemental compositions wereimplemented into the material card of the MCNPX code.The source regions were defined separately in each run:

thyroid, stomach (stomach wall and contents), small intestine(small intestine wall and contents), kidneys, liver, bladder

contents, salivary glands, maternal remaining tissues, fetalthyroid and remaining tissues. Gastrointestinal contents(apart from stomach and small intestine contents) were notincluded in source regions. Next, the total organ doses result-ing from photons and electrons emitted from ten sourceregions were obtained.The simulations provided the dose (MeV/g), i.e. energy

deposition (MeV) per unit mass (g), in each target organ (T)per emitted particle. The dose per particle was multiplied bythe total photon or electron yield per decay and summed toobtain the absorbed dose (mGy/MBq). The dose was scoredusing the track length estimate of the heating tally (F6) forphotons (kerma approximation) and energy deposition (+F6)for electrons. Photons and electrons (mode P E) were trans-ported when the emitted particle was an electron, while onlyphotons were transported (mode P) for photons emitted fromthe source region.The spectra published in ENSDF decay data [14] with

yields >0.1% was employed for the estimation of the organdoses. The beta spectrum of 131I was approximated using theFermi function with respect to the maximum energy of beta.The auger electrons were determined by their averageenergy, and the conversion electrons by their maximumenergy in subshells. Absorbed doses to active red marrowand the endosteal region (bone surface) were estimated byusing the F6 tally and the masses of active marrow in bonesites, as published in ICRP Publication 116 [15]. For a moredetailed investigation, additional MCNPX mesh tallies wereused to graphically display the dose in voxels of the model insagittal view. Mesh tally type 1 with the ‘pedep’ option andtype 3 with the ‘total’ option were used for photon and elec-tron sources, respectively, which score the average energydeposition per unit volume (MeV/cm3/source-particle). Thevisualization was performed in two arrays of 159 × 348crossing at the i = 149 and i = 176 of the phantom. A rect-angular grid exactly overlaid on the lattice geometry wasdefined. Therefore, the energy deposited per unit volume ineach voxel could be converted to the energy per unit mass bydividing by each voxel’s density. The energy deposited perunit mass (MeV/g/source-particle) in each voxel was thenconverted to the absorbed dose (mGy/MBq) for visualizationof the dose distribution map.

RESULTS

Organs masses vs reference dataThe organ masses after the voxelization process of the adultfemale ICRP reference phantom (AF) [16] and the pregnantmodel developed in this project were compared in Table 1.The total mass of the model, which was calculated frommultiplying the voxel volume by its density, was 68.1 kgwhile for the AF phantom it was 60.0 kg. Considering thefact that much effort was taken to adjust replaced organmasses to the reference values obtained during 3D modeling,

Development of a 9-months pregnant hybrid phantom 733

the differences are obviously low in most cases. Certainorgans, such as the uterus, breasts, skin and adipose tissuewill have increased masses during pregnancy, as indicated inthis table, while the gastrointestinal contents have decreasedin volume. The discrepancies in other organs are <1%,except in the gall bladder wall and the trunk muscles, whichhave discrepancies of <3%. ICRP Publication 89 referencemasses were carefully adopted for fetal organs, and the dif-ferences are <0.3% (see Table 1). The total mass of the fetusis 3.471 kg, which differs from the reference value of 3.5 kgby ~0.8%. The mass of the uteroplacental unit and itscontents is 4.8 kg.The soft tissue densities and material compositions of the

current pregnant phantom were primarily adapted from ICRPPublication 110 [16], which provides these data for the adultfemale ICRP reference phantom. In the case of fetal tissues,suitable surrogates were adopted. The ‘Density and Materialcomment’ columns provide a numerical key to data references(e.g. fetal soft tissue density was taken from Maynard et al.,while material composition was taken from Xu et al.) (seeTable 1 and its footnotes for further details).

Cumulated activities for pregnant female modelThe calculation of radiation dose estimates to the fetus is im-portant for radiation protection purposes. To obtain the bestestimates of the radiation dose to the fetus, the best biologicaland physical models should be employed. In this paper, thebest available biokinetic model [4, 11] for a pregnant womanwas used to calculate cumulated activities for maternalthyroid, salivary glands, stomach, small intestine, liver,bladder contents, kidneys, feces and remaining tissues, inaddition to fetal thyroid and the remaining fetal organs. Thebiokinetic model was employed assuming a 5, 15, 25, 35,45, 55 and 95% maximum thyroid uptake level. Tables 2 and3 show the cumulated activities calculated in this investiga-tion in comparison with those published by two other refer-ences [4, 10]. ICRP Publication 53 [10] provides cumulatedactivities for normal patients at six different maximumthyroid uptakes within the range of 5–55%. A 95% thyroiduptake was added in this study to include hyperthyroidpatients. This model applies to the intravenous administrationof iodide; however, the oral administration of iodide willdelay the appearance of iodide in the blood by 10–15 min,which will have only a minimal effect on the activity level inthe blood and little effect on the thyroid uptake [10]. Thiscompartmental biokinetic model includes fetal thyroid andfetal organic compartments. The model applied in this study,describes the behavior of administered iodide and organical-ly bound iodine (in T3 and T4 form), which is released to thebody tissues from the thyroid, while dose estimates reportedby ICRP Publication 53 [10] do not consider the effects oforganically bound iodine. In this paper, in addition to othersource organs, the activities cumulated in salivary glandsand in the fetal thyroid were assessed separately, whereas

previous publications [4, 10] do not account for these vitalorgans in dose estimates. This comparison is presented inTables 2 and 3.The biokinetic model was not provided for Tc-99m per-

technetate in Russell et al. [4, 11] or in ICRP Publication53 [10], so the cumulated activities were not computed in thispaper separately but taken from those references (Table 4).

Internal dose estimatesOrgan doses from 131I and 123I distributed within a 9-monthpregnant patient were calculated at seven levels [5, 15, 25,35, 45, 55 and (extremely) 95%] of thyroid uptake andpresented in Tables 5 and 6. In addition, organ doses wereobtained for the present model, by using cumulated activitiesfrom two other references [4, 10]. In the last column ofthe Tables 5 and 6, the reported dose to the 9-month fetusand fetal thyroid for the stylized model were also tabulated[4, 17].Not all maternal organ doses were addressed here; only

doses of critical maternal organs and fetal organs were consid-ered in this research. As expected, the thyroid gland receivesthe largest maternal organ dose for 131I and 123I. The fetal radi-ation dose for this pregnant model at 25% uptake was esti-mated at 2.55 × 10−1 and 2.40 × 10−1 mGy/MBq, by using thecumulated activities of this study and Russell et al., respective-ly, while this value was reported as 2.70 × 10−1 mGy/MBq forthe stylized model (Table 5). This demonstrates that the pub-lished fetal dose has been overestimated in the mathematicalmodel.Fetal radiation doses were quantified for 20 different

organs separately. As an example, the fetal thyroid dose was349 mGy/MBq when assuming a maternal thyroid uptake of25%, in comparison with the figure of 270 mGy/MBqreported for the mathematical version [4] (see Table 5).These comparisons reveal an underestimation of ~25% of theabsorbed dose to the fetal thyroid as reported for the stylizedphantom compared with the values based on the currenthybrid phantom.It should be noted that organ doses have considerable var-

iations due to different thyroid uptake. This issue has beenaddressed earlier for the adult female ICRP voxel phantom[18]. Of particular importance is the fetal thyroid dose,which decreases by a factor of 15 as a result of increased ma-ternal thyroid uptake.To better clarify dose variations due to different cumulated

activities, distributions of the dose in the pregnant womanand her fetus were computed and plotted in Figs 4–6. This2D mesh tally provided the dose distribution map of two sa-gittal planes of the phantom, including either maternal orfetal thyroid voxels (Fig. 6). The positions and sizes of meshtallies were carefully selected so that only one material wasincluded in each mesh tally. Then, absorbed dose per unit ac-tivity administered to the mother was calculated for all thevoxels, and the dose distribution was plotted. Figures 4–6

E. Hoseinian-Azghadi et al.734

Table 1. Organ masses of adult female ICRP Reference voxel phantom in comparison with the current model and their differences

Organs of interestICRP 110 Reference

mass (g)New mass

(g)Difference

(%)Densitycomment

Materialcomment

Cartilage, trunk 313.61 313.61 0.00 1 1

Breast, left, adipose tissue 150.00 489.73 226.48 1 1

Breast, right, adipose tissue 150.00 512.11 241.40 1 1

Gall bladder wall 10.24 9.96 −2.74 1 1

Gall bladder contents 45.75 31.82 −30.45 1 1

Stomach wall 140.00 140.00 0.00 1 1

Stomach contents 230.01 165.84 −27.90 1 1

Small intestine wall 600.00 600.00 0.00 1 1

Small intestine contents 280.01 156.61 −44.07 1 1

Ascending colon wall 90.00 90.00 0.00 1 1

Ascending colon contents 100.01 86.42 −13.59 1 1

Transverse colon wall, right 55.00 55.00 0.01 1 1

Transverse colon contents, right 60.00 58.06 −3.23 1 1

Transverse colon wall, left 55.00 54.79 −0.37 1 1

Transverse colon contents, left 30.00 28.51 −4.96 1 1

Descending colon wall 90.00 90.00 0.00 1 1

Descending colon contents 50.00 49.00 −2.03 1 1

Sigmoid colon wall 45.01 45.01 −0.01 1 1

Sigmoid colon contents 80.00 79.01 −1.23 1 1

Rectum wall 25.00 24.85 −0.56 1 1

Kidney, left, pelvis 149.48 149.48 0.00 1 1

Kidney, right, pelvis 125.53 125.53 0.00 1 1

Liver 1 400.00 1 399.72 −0.02 1 1

Lung, left, tissue 377.02 377.02 0.00 1 1

Lung, right, tissue 472.03 471.85 −0.04 1 1

Muscle, trunk 8 518.22 8 389.34 −1.51 1 1

Esophagus (wall) 34.99 34.99 0.01 1 1

Ovary, left 5.50 5.50 0.03 1 1

Ovary, right 5.50 5.50 0.03 1 1

Pancreas 120.00 120.00 0.00 1 1

Residual tissue, trunk 11 803.12 14 149.25 19.88 1 1

Skin, trunk 1 004.12 1 114.05 10.95 1 1

Spleen 130.00 130.00 0.00 1 1

Urinary bladder wall 40.00 40.00 −0.01 1 1

Thyroid 17.00 17.00 0.00

Urinary bladder contents 200.00 226.53 13.26 1 1

Uterus 79.99 823.33 929.30 1 1

Continued

Development of a 9-months pregnant hybrid phantom 735

indicates a comparison between the dose distributions calcu-lated with different biokinetic data. The comparison of dosedistributions for three different sets of cumulated activities(this study, Russell et al. and ICRP Publication 53) at 25%thyroid uptake are shown in Fig. 4. In Fig. 5, a comparisonbetween different thyroid uptakes has been made for 131Iincorporation.

For the case of Tc-99-m pertechnetate, organ doses havebeen tabulated in Table 7. The first and second columns indi-cate the absorbed dose calculated for the current pregnantmodel by using cumulated activities from Russell et al. andICRP Publication 53. The third column illustrates the fetaldose reported by Russell et al. The fetal dose was estimatedat 9.31 × 10−3 mGy/MBq for this pregnant model, which was

Table 1. Continued

Organs of interestICRP 89 Reference mass

(g)New mass

(g)Difference

(%)Densitycomment

Materialcomment

Amniotic fluid 207.17 2* 7

Umbilical cord 25.60 1* 1*

Placenta 1 070.39 3 6

Fetus 3 500 3 471.98 0.80

Fetus, brain 370 370.00 0.00 2, 5 5

Fetus, skeleton 204.31 2 4

Fetus, eyes 4.13 2 1

Fetus, spinal cord 5.43 2 1

Fetus, thyroid 1.3 1.30 0.24 2 1

Fetus, lungs 60 59.87 0.21 2 1

Fetus, thymus 13 13.00 −0.03 2 1

Fetus, heart 20 20.00 0.01 2 1

Fetus, liver 130 130.30 −0.23 2 1

Fetus, kidneys 25 25.00 −0.02 2 1

Fetus, adrenals 6 6.00 0.00 2 1

Fetus, spleen 9.5 9.50 0.00 2 1

Fetus, SI wall 13.63 2 1

Fetus, SI contents 11.39 2 1

Fetus, LI wall 6.03 2 1

Fetus, LI contents 5.95 2 1

Fetus, bladder wall 9.44 2 1

Fetus, bladder contents 27.22 2 1

Fetus, stomach wall 10.15 2 1

Fetus, stomach contents 3.86 2 1

Fetus, gall bladder wall 0.72 2 1

Fetus, gall bladder contents 0.72 2 1

Fetus, pancreas 5 4.99 0.11 2 1

Fetus, soft tissue 2 299.96 2 3

Fetus, skin 229.08 1 1

1ICRP 110: adult female densities and materials of given organ or tissue. The last two columns are comments regarding tissuecompositions and mass densities. 1*ICRP 110: density and material of blood. 2Maynard et al.: 38 weeks densities and materials ofgiven organ or tissue. 2*Maynard et al.: density of gastrointestinal contents. 3Xu et al.: density and material of fetus soft tissue. 4Xuet al.: density and material of skeleton. 5Xu et al.: density and material of brain. 6Xu et al.: density and material of placenta. 7Xu et al.:density and material of uterine contents.

E. Hoseinian-Azghadi et al.736

in good agreement with that of the stylized model [4]. Thedose distributions are plotted in Fig. 6 for both sets of cumu-lated activities.

DISCUSSION

The current pregnant model is the first phantom that repre-sents the whole body of a pregnant female and about 20 ofthe fetal organs. In addition, variations in the shape andvolume of the soft tissues in the abdominopelvic cavity havebeen modeled as realistically as possible, according to MRimages. The boundary representation method was used toform the current phantom. It demonstrates the ability of thehybrid construction process to model maternal and fetalanatomy from medical image sets with sufficient imagequality. Construction of this model without using the hybridapproach would have been impossible due to the followingrequirements: (i) replacing the abdominal and pelvic regionof the female ICRP reference voxel phantom with that of thepregnant model and matching the edges of overlappingorgans; (ii) modeling the fetal skeletal bones and thyroid,which were poorly or not recognizable in the original MRimage sets, in their correct anatomical location; (iii) impart-ing deformability to certain structures such as the liver,stomach, spleen, pancreas and gall bladder.Biokinetic data during pregnancy were needed to calculate

organ doses. Activities cumulated in the fetus and in the ma-ternal source organs were published by Russell et al., butthey did not mention the fetal thyroid as a source organbecause the stylized version of the pregnant phantom doesnot contain this organ. With regard to dose calculation, thistiny organ of 1.3 g mass is very important since fetal thyroidconcentrates the iodine, which crosses the placenta. The fetalcumulated activity (4.72 h) reported by Russell et al. is ingood agreement with the sum of those activities given in thisstudy for both fetal thyroid and the remaining fetal organs(4.42 h + 0.31 h). It should be noted that 94% of cumulatedactivity assigned to the fetus belongs to the fetal thyroid.Assuming a uniformly distributed source in the fetus body,rather than a local source in the fetal thyroid, caused doseunderestimation of ~7%. The fetal dose was 2.55 × 10−1

mGy/MBq when the fetal thyroid was defined as a separatesource compared with 2.40 × 10−1 mGy/MBq when the totalfetus body considered as a uniform source. This occursbecause more radiation would escaped from the fetus bodywhen it was defined as a uniform source organ. More signifi-cant disagreements of about three orders of magnitude wereobserved for the fetal thyroid dose due to self-irradiation(see Tables 5 and 6). The absorbed dose to adjacent organsto the fetal thyroid, such as the brain, appears to increase bya factor of 3 when the source is defined locally in thethyroid. This issue is clearly indicated in the dose mapplotted in Fig. 4.

Tab

le2.

Cum

ulated

activ

ities

for1

31Isodium

iodide

obtained

inthisstudyto

estim

ateinternaldosesin

comparisonwith

twootherreferences

131 I

Thisstud

yRusselletal.

ICRP53

5%15%

25%

35%

45%

55%

95%

25%

25%

Thyroid

12.27h

1.55

d2.61

d3.73

d4.64

d5.76

d10.41d

2.54

d2.53

d

Salivaryglands

32.20min

29.04min

25.84min

22.42min

19.67min

16.27min

2.14

min

Stomach

1.61

h1.45

h1.29

h1.12

h59.00min

48.82min

6.43

min

1.33

h1.66

h

SI

1.82

h1.65

h1.46

h1.27

h1.11

h55.33min

7.29

min

1.5h

1.66

h

Liver

51.37min

1.16

h1.46

h1.79

h2.05

h2.37

h3.71

h1.16

h

Bladdercontents

1.67

h1.50

h1.33

h1.15

h1.01

h50.64min

24.74min

1.92

h1.32

h

Kidney

4.80

min

4.80

min

4.80

min

4.80

min

4.80

min

4.80

min

4.80

min

5.7min

Feces

<30

s<30

s<30

s<30

s<30

s<30

s<30

s

Rem

aining

tissues

6.36

h6.33

h6.30

h6.27

h6.25

h6.21

h6.08

h6.4h

7.72

h

Fetus–thyroid

5.50

h4.96

h4.42

h3.83

h3.36

h2.78

h21.99min

4.72

h

Rem

aining–Fetus

23.09min

20.82min

18.53min

16.07min

14.11min

11.67min

1.54

min

Development of a 9-months pregnant hybrid phantom 737

Variations due to different uptakes are shown in Fig. 5. Ingeneral, organ doses close to the urinary bladder, and par-ticularly fetus doses, tend to decrease throughout the uptakerange. This occurs because the increase in maternal thyroiduptake causes radioiodine cumulated in the other organs todecline. The 95% uptake of maternal thyroid extreme par-ticularly demonstrates this fact. This level of uptake was con-sidered since fetal dose has not been well established forpatients whose iodine kinetics differ from the standard

model. Conversely, as thyroid uptake varies from 0% to95%, the thyroid gland takes up more radioiodine; hence, theabsorbed dose to the thyroid and its adjacent organsincreases.Discrepancies between organ doses with similar thyroid

uptake should assign to different biokinetic data utilized inthe calculations. For a thyroid uptake of 25%, a comparisonwas made between organ doses assessed using three differentsets of cumulated activities. For example, the thyroid doses

Table 3. Cumulated activities for 123I sodium iodide obtained in this study to estimate internal doses in comparison with two otherreferences

123I This study Russellet al.

ICRP53

5% 15% 25% 35% 45% 55% 95% 25% 25%

Thyroid 37.94 min 1.97 h 3.41 h 5.04 h 6.43 h 8.25 h 17.37 h 3.28 h 2.94 h

Salivary glands 21.00 min 19.49 min 17.84 min 15.98 min 14.39 min 12.31 min 1.89 min

Stomach 1.05 h 58.46 min 53.53 min 47.94 min 43.17 min 36.93 min 5.66 min 54.42 min 1.08 h

SI 1.19 h 1.10 h 1.01 h 54.33 min 48.92 min 41.85 min 6.42 min 1.03 h 1.08 h

Liver 26.57 min 24.82 min 22.94 min 20.81 min 19.00 min 16.66 min 4.72 min 6.78 min

Bladder contents 1.06 h 58.98 min 54.35 min 49.08 min 44.58 min 38.58 min 8.46 min 1.25 h 50 min

Kidney 3.39 min 3.39 min 3.39 min 3.39 min 3.39 min 3.39 min 3.39 min 3.7 min

Feces <0.01 s <0.01 s <0.01 s <0.01 s <0.01 s <0.01 s <0.01 s

Remainingtissues

3.98 h 3.69 h 3.39 h 3.04 h 2.75 h 2.36 h 25.53 2.92 h 5.03 h

Fetus–thyroid 19.04 min 17.65 min 16.16 min 14.47 min 13.03 min 11.15 min 1.71 min 16.26 min

Remaining–fetus 19.80 s 18.40 s 16.85 s 15.08 s 13.57 s 11.62 s 1.78 s

Table 4. Cumulated activities for Tc-99m sodium pertechnetate from two references used in this study to estimate internal doses

Tc-99m Russell et al. ICRP 53

Thyroid 2.28 min 2.23 min

Salivary glands 3.35 min

Stomach wall 18.18 min 14.9 min

Stomach contents 9.24 min

SI contents 25.3 min

Right colon = ULI wall 47.40 min 32.6 mincontents 44.6 min

Left colon = LLI wall 29.40 mincontents 21.8 min

Kidneys 2.00 min

Bladder contents 47.70 min 20.7 min

Remaining tissues 5.02 h 4.32 h

Fetus 20.04 min

Placenta 57.24 min

E. Hoseinian-Azghadi et al.738

Table 5. Organ doses (mGy/MBq) calculated for this model with different sets of cumulated activities, in addition to fetal doses reported by previous publications

131I Sodium Iodide This study a b

Thyroid uptake 5% 15% 25% 35% 45% 55% 95% 25% 25%

Active red marrow 7.55E-02 1.48E-01 2.22E-01 3.01E-01 3.64E-01 4.42E-01 7.68E-01 2.17E-01 2.17E-01

Colon 6.38E-02 6.46E-02 6.55E-02 6.64E-02 6.71E-02 6.81E-02 7.25E-02 6.51E-02 6.59E-02

Lungs 9.35E-02 1.86E-01 2.79E-01 3.79E-01 4.60E-01 5.59E-01 9.72E-01 2.73E-01 2.77E-01

Stomach wall 7.12E-01 6.55E-01 5.97E-01 5.36E-01 4.86E-01 4.25E-01 1.70E-01 6.14E-01 7.48E-01

Breasts 4.69E-02 8.63E-02 1.26E-01 1.69E-01 2.03E-01 2.46E-01 4.22E-01 1.24E-01 1.25E-01

Ovaries 6.46E-02 6.04E-02 5.63E-02 5.20E-02 4.85E-02 4.44E-02 3.29E-02 6.99E-02 6.14E-02

Urinary bladder wall 1.52E-01 1.39E-01 1.26E-01 1.12E-01 1.01E-01 8.77E-02 5.25E-02 1.72E-01 1.32E-01

Esophagus 4.30E-01 1.23E + 00 2.05E + 00 2.92E + 00 3.62E + 00 4.49E + 00 8.09E + 00 2.00E + 00 1.99E + 00

Liver 1.20E-01 1.64E-01 2.09E-01 2.57E-01 2.96E-01 3.44E-01 5.43E-01 1.81E-01 7.95E-02

Thyroid 8.17E + 01 2.48E + 02 4.17E + 02 5.97E + 02 7.42E + 02 9.20E + 02 1.66E + 03 4.06E + 02 4.04E + 02

Endosteal region 4.36E-02 7.63E-02 1.10E-01 1.45E-01 1.73E-01 2.09E-01 3.55E-01 1.07E-01 1.09E-01

Salivary gland 9.72E-01 1.04E + 00 1.10E + 00 1.17E + 00 1.23E + 00 1.30E + 00 1.58E + 00 3.77E-01 3.76E-01

Adrenal 6.94E-02 7.89E-02 8.86E-02 9.90E-02 1.07E-01 1.18E-01 1.61E-01 8.68E-02 8.17E-02

Kidney 9.59E-02 9.86E-02 1.01E-01 1.04E-01 1.07E-01 1.10E-01 1.22E-01 6.45E-02 1.10E-01

Pancreas 9.61E-02 9.77E-02 9.93E-02 1.01E-01 1.02E-01 1.04E-01 1.11E-01 1.02E-01 1.09E-01

SI-wall 3.48E-01 3.18E-01 2.88E-01 2.57E-01 2.31E-01 1.99E-01 6.82E-02 2.92E-01 3.21E-01

Spleen 8.04E-02 8.67E-02 9.31E-02 9.99E-02 1.05E-01 1.12E-01 1.40E-01 9.21E-02 1.02E-01

Thymus 3.62E-01 1.05E + 00 1.74E + 00 2.49E + 00 3.08E + 00 3.82E + 00 6.89E + 00 1.70E + 00 1.70E + 00

Uterus 7.66E-02 7.32E-02 6.98E-02 6.62E-02 6.34E-02 5.98E-02 4.65E-02 7.27E-02 7.34E-02

Remainder tissues 1.32E-01 2.41E-01 3.51E-01 4.69E-01 5.63E-01 6.80E-01 1.17E + 00 3.41E-01 3.47E-01

Continued

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Table 5. Continued

131I Sodium Iodide This study a b c

Thyroid uptake 5% 15% 25% 35% 45% 55% 95% 25% 25% 25%

Placenta 5.79E-02 5.50E-02 5.22E-02 4.91E-02 4.67E-02 4.37E-02 3.31E-02 6.58E-02 6.53E-02

Fetus 3.08E-01 2.81E-01 2.55E-01 2.26E-01 2.03E-01 1.75E-01 5.85E-02 2.41E-01 2.40E-01 2.70E-01

Fetus, brain 1.11E-01 1.02E-01 9.28E-02 8.28E-02 7.47E-02 6.48E-02 2.49E-02 2.43E-01 2.41E-01

Fetus, skeleton 1.35E-01 1.27E-01 1.19E-01 1.10E-01 1.03E-01 9.45E-02 5.88E-02 2.32E-01 2.32E-01

Fetus, eyes 1.08E-01 9.95E-02 9.12E-02 8.23E-02 7.51E-02 6.63E-02 3.01E-02 2.48E-01 2.48E-01

Fetus, spinal cord 3.65E-01 3.32E-01 2.99E-01 2.64E-01 2.36E-01 2.01E-01 5.68E-02 2.46E-01 2.48E-01

Fetus, thyroid 4.35E + 02 3.92E + 02 3.49E + 02 3.03E + 02 2.65E + 02 2.20E + 02 2.90E + 01 1.12E-01 1.13E-01 2.70E + 02

Fetus, Lungs 2.07E-01 1.91E-01 1.74E-01 1.56E-01 1.41E-01 1.23E-01 4.92E-02 3.50E-01 3.52E-01

Fetus, thymus 1.25E + 00 1.13E + 00 1.01E + 00 8.79E-01 7.75E-01 6.46E-01 1.12E-01 2.50E-01 2.51E-01

Fetus, heart 2.18E-01 2.01E-01 1.83E-01 1.64E-01 1.49E-01 1.30E-01 5.16E-02 2.59E-01 2.60E-01

Fetus, Liver 9.32E-02 8.83E-02 8.33E-02 7.80E-02 7.38E-02 6.85E-02 4.68E-02 2.52E-01 2.53E-01

Fetus, kidneys 7.67E-02 7.56E-02 7.44E-02 7.32E-02 7.22E-02 7.10E-02 6.61E-02 2.55E-01 2.56E-01

Fetus, adrenals 9.37E-02 8.96E-02 8.55E-02 8.11E-02 7.75E-02 7.31E-02 5.49E-02 2.58E-01 2.60E-01

Fetus, spleen 1.08E-01 1.03E-01 9.87E-02 9.39E-02 9.00E-02 8.52E-02 6.54E-02 2.66E-01 2.70E-01

Fetus, SI wall 8.51E-02 8.41E-02 8.31E-02 8.20E-02 8.12E-02 8.01E-02 7.59E-02 2.63E-01 2.63E-01

Fetus, LI wall 6.58E-02 6.67E-02 6.77E-02 6.88E-02 6.96E-02 7.06E-02 7.51E-02 2.54E-01 2.54E-01

Fetus, bladder wall 6.46E-02 6.48E-02 6.51E-02 6.53E-02 6.55E-02 6.58E-02 6.70E-02 2.55E-01 2.54E-01

Fetus, stomach wall 1.15E-01 1.09E-01 1.03E-01 9.68E-02 9.17E-02 8.53E-02 5.90E-02 2.63E-01 2.65E-01

Fetus, gall bladder wall 7.52E-02 7.23E-02 6.94E-02 6.63E-02 6.39E-02 6.08E-02 4.82E-02 2.52E-01 2.52E-01

Fetus, pancreas 9.86E-02 9.51E-02 9.16E-02 8.78E-02 8.48E-02 8.10E-02 6.55E-02 2.65E-01 2.67E-01

Fetus, soft tissue 1.56E-01 1.45E-01 1.33E-01 1.21E-01 1.11E-01 9.92E-02 4.93E-02 2.40E-01 2.39E-01

Fetus, skin 7.77E-02 7.41E-02 7.04E-02 6.65E-02 6.34E-02 5.95E-02 4.41E-02 2.07E-01 2.07E-01

a,bOrgan dose estimates using cumulated activities from Russell et al. and ICRP Publication 53, respectively. cDose to the fetus and fetal thyroid reported by Russell et al. [4]and [17], respectively.

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Table 6. Organ doses (mGy/MBq) calculated for this model with different sets of cumulated activities, in addition to fetal doses reported by previous publications

123I Sodium Iodide This study a b

Thyroid uptake 5% 15% 25% 35% 45% 55% 95% 25% 25%

Active red marrow 8.87E-03 1.05E-02 1.22E-02 1.42E-02 1.59E-02 1.81E-02 2.91E-02 1.14E-02 1.25E-02

Colon 1.19E-02 1.11E-02 1.03E-02 9.34E-03 8.54E-03 7.50E-03 2.25E-03 9.70E-03 1.15E-02

Lungs 9.73E-03 1.16E-02 1.35E-02 1.58E-02 1.77E-02 2.02E-02 3.26E-02 1.20E-02 1.46E-02

Stomach wall 1.02E-01 9.49E-02 8.72E-02 7.84E-02 7.09E-02 6.11E-02 1.21E-02 8.75E-02 1.04E-01

Breasts 5.20E-03 5.66E-03 6.15E-03 6.72E-03 7.20E-03 7.83E-03 1.10E-02 5.34E-03 6.83E-03

Ovaries 1.81E-02 1.68E-02 1.55E-02 1.40E-02 1.27E-02 1.10E-02 2.31E-03 1.93E-02 1.64E-02

Urinary bladder wall 3.81E-02 3.55E-02 3.27E-02 2.95E-02 2.68E-02 2.32E-02 5.02E-03 4.34E-02 3.19E-02

Esophagus 1.94E-02 4.40E-02 7.05E-02 1.00E-01 1.26E-01 1.59E-01 3.27E-01 6.67E-02 6.26E-02

Liver 1.76E-02 1.67E-02 1.58E-02 1.48E-02 1.39E-02 1.27E-02 6.85E-03 8.61E-03 9.47E-03

Thyroid 7.47E-01 2.32E + 00 4.01E + 00 5.93E + 00 7.56E + 00 9.70E + 00 2.04E + 01 3.86E + 00 3.46E + 00

Endosteal region 5.85E-03 6.43E-03 7.05E-03 7.75E-03 8.35E-03 9.14E-03 1.31E-02 6.43E-03 7.68E-03

Salivary gland 1.08E-01 1.04E-01 1.00E-01 9.50E-02 9.08E-02 8.53E-02 5.76E-02 1.03E-02 1.02E-02

Adrenal 1.53E-02 1.44E-02 1.34E-02 1.24E-02 1.15E-02 1.03E-02 4.29E-03 1.08E-02 1.40E-02

Kidney 1.74E-02 1.66E-02 1.58E-02 1.49E-02 1.41E-02 1.30E-02 7.84E-03 9.47E-03 1.79E-02

Pancreas 2.37E-02 2.22E-02 2.05E-02 1.86E-02 1.70E-02 1.49E-02 4.46E-03 1.91E-02 2.37E-02

SI-wall 4.93E-02 4.58E-02 4.20E-02 3.77E-02 3.41E-02 2.93E-02 5.36E-03 4.22E-02 4.57E-02

Spleen 2.19E-02 2.05E-02 1.91E-02 1.75E-02 1.61E-02 1.43E-02 5.16E-03 1.84E-02 2.28E-02

Thymus 1.50E-02 3.62E-02 5.91E-02 8.49E-02 1.07E-01 1.36E-01 2.80E-01 5.61E-02 5.26E-02

Uterus 1.09E-02 1.02E-02 9.35E-03 8.45E-03 7.68E-03 6.67E-03 1.61E-03 9.67E-03 1.06E-02

Remainder tissues 1.62E-02 1.84E-02 2.07E-02 2.33E-02 2.56E-02 2.85E-02 4.33E-02 1.85E-02 2.07E-02

Continued

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phantom741

Table 6. Continued

123I Sodium Iodide This study a b c

Thyroid uptake 5% 15% 25% 35% 45% 55% 95% 25% 25% 25%

Amniotic fluid 9.06E-03 8.45E-03 7.81E-03 7.07E-03 6.44E-03 5.62E-03 1.48E-03 8.53E-03 9.20E-03

Umblical cord 6.24E-03 5.87E-03 5.47E-03 5.02E-03 4.63E-03 4.13E-03 1.59E-03 5.92E-03 7.49E-03

Placenta 8.81E-03 8.21E-03 7.56E-03 6.82E-03 6.19E-03 5.36E-03 1.20E-03 8.69E-03 8.94E-03

Fetus 1.15E-02 1.07E-02 9.88E-03 8.93E-03 8.12E-03 7.07E-03 1.77E-03 9.24E-03 1.00E-02 9.80E-03

Fetus, brain 7.74E-03 7.20E-03 6.63E-03 5.97E-03 5.42E-03 4.68E-03 1.02E-03 8.67E-03 8.75E-03

Fetus, skeleton 2.06E-02 1.92E-02 1.77E-02 1.61E-02 1.46E-02 1.28E-02 3.40E-03 1.81E-02 2.00E-02

Fetus, eyes 6.31E-03 5.89E-03 5.43E-03 4.90E-03 4.46E-03 3.87E-03 9.45E-04 7.40E-03 7.97E-03

Fetus, spinal cord 1.87E-02 1.74E-02 1.60E-02 1.44E-02 1.30E-02 1.13E-02 2.28E-03 9.54E-03 1.06E-02

Fetus, thyroid 4.12E + 00 3.82E + 00 3.50E + 00 3.14E + 00 2.82E + 00 2.42E + 00 3.71E-01 7.04E-03 7.80E-03 2.90E + 00

Fetus, lungs 1.38E-02 1.28E-02 1.18E-02 1.07E-02 9.66E-03 8.37E-03 1.90E-03 1.15E-02 1.27E-02

Fetus, thymus 4.10E-02 3.80E-02 3.49E-02 3.13E-02 2.82E-02 2.42E-02 4.20E-03 8.99E-03 9.92E-03

Fetus, heart 1.28E-02 1.19E-02 1.10E-02 9.91E-03 9.00E-03 7.81E-03 1.85E-03 9.61E-03 1.07E-02

Fetus, liver 7.01E-03 6.56E-03 6.07E-03 5.52E-03 5.05E-03 4.44E-03 1.37E-03 8.15E-03 9.04E-03

Fetus, kidneys 1.02E-02 9.54E-03 8.85E-03 8.06E-03 7.40E-03 6.52E-03 2.14E-03 1.11E-02 1.26E-02

Fetus, adrenals 1.18E-02 1.11E-02 1.02E-02 9.25E-03 8.43E-03 7.36E-03 2.01E-03 1.23E-02 1.38E-02

Fetus, spleen 1.58E-02 1.47E-02 1.36E-02 1.23E-02 1.12E-02 9.80E-03 2.63E-03 1.55E-02 1.77E-02

Fetus, SI wall 1.08E-02 1.01E-02 9.38E-03 8.56E-03 7.87E-03 6.96E-03 2.40E-03 1.14E-02 1.30E-02

Fetus, LI wall 7.54E-03 7.09E-03 6.62E-03 6.08E-03 5.62E-03 5.02E-03 2.00E-03 8.94E-03 1.01E-02

Fetus, bladder wall 6.23E-03 5.87E-03 5.48E-03 5.05E-03 4.68E-03 4.19E-03 1.75E-03 8.11E-03 9.05E-03

Fetus, stomach wall 1.17E-02 1.09E-02 1.01E-02 9.15E-03 8.36E-03 7.32E-03 2.11E-03 1.17E-02 1.33E-02

Fetus, gall bladder wall 5.96E-03 5.59E-03 5.19E-03 4.74E-03 4.35E-03 3.85E-03 1.32E-03 7.73E-03 8.56E-03

Fetus, pancreas 1.26E-02 1.18E-02 1.09E-02 9.87E-03 9.02E-03 7.91E-03 2.34E-03 1.29E-02 1.47E-02

Fetus, soft tissue 9.39E-03 8.76E-03 8.09E-03 7.33E-03 6.68E-03 5.83E-03 1.56E-03 8.53E-03 9.26E-03

Fetus, skin 8.40E-03 7.85E-03 7.25E-03 6.58E-03 6.00E-03 5.25E-03 1.46E-03 8.75E-03 9.65E-03

a, bOrgan dose estimates using cumulated activities from Russell et al. and ICRP Publication 53, respectively. cDose to the fetus and fetal thyroid reported by Russell et al. [4]and [17], respectively.

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Fig. 4. Comparison of dose maps calculated with three different sets of cumulated activities at 25% thyroid uptake.Both photons and electrons have been included in the calculations.

Fig. 5. Comparison of dose maps of two sagittal planes of the phantom calculated at different maternal thyroiduptakes, including both photons and electrons contributions of 131I. Upper plots include fetal thyroid voxels and lowerplots contain maternal thyroid voxels.

Development of a 9-months pregnant hybrid phantom 743

in 131I sodium iodide incorporation are 4.17 × 102,4.06 × 102 and 4.04 × 102 mGy/MBq calculated using threesets of cumulated activities (this study, Russell et al. andICRP 53, respectively). On the other hand, the values ofcumulated activity assigned to the thyroid are 2.61, 2.54 and2.53 d, respectively (see Table 2). It is clear that there is a re-lationship between organ doses and cumulated activities.Indeed, the absorbed dose to source organs is larger whenthe cumulated activity assigned to the organ is higher.For organs other than source regions, the nearest source

region contributes mostly in organ dose, and its cumulatedactivity should be considered for the interpretation of results.As an example, absorbed doses to the ovaries for 123I incorp-oration calculated using three sets of cumulated activities are1.55 × 10−2, 1.93 × 10−2 and 1.64 × 10−2 mGy/MBq, respect-ively. The values for the urinary bladder content (which is inthe vicinity of the target organ) are 54.35 min, 1.25 h and50 min, separately (see Table 3). It was expected that the

lowest dose absorbed would be obtained using the third set(ICRP Publication 53), but according to the result(1.55 × 10−2) the minimum value was obtained using the firstset of cumulated activities. This is due the fact that in the thirdset the fetus was defined as a uniform source organ, and as thedistance between the source (fetus) and the target (ovary) thusdecreased, absorbed dose to the ovaries increased.In the case of Tc-99-m pertechnetate, good agreement was

achieved for fetal dose between our calculations and those ofRussell et al. [4], although the fetal organ doses are newlyderived in this study. The dose distributions plotted in Fig. 6reveal a ‘hot’ area in the placental region. This can beexplained by the fact that the cumulated activity assigned tothe placenta was greater than normalized cumulated activityof the remaining tissues. Fetal bones are distinguishable inthis plot, which means that the absorbed dose to the fetalbones is higher than that of the fetal soft tissues as a result oftheir greater density.

Table 7. Organ doses (mGy/MBq) calculated for this model with different sets of cumulated activities, in addition to fetal dosesreported by previous publications

99m-Tc Sodium Pertechnetate a b a b c

Active red marrow 5.28E-03 4.88E-03 Placenta 1.93E-02 1.85E-02

Colon 2.95E-02 2.76E-02 Fetus 8.81E-03 9.48E-03 9.30E-03

Lungs 5.16E-03 4.90E-03 Fetus, brain 8.92E-03 8.34E-03

Stomach wall 1.92E-02 2.93E-02 Fetus, skeleton 1.56E-02 1.70E-02

Breasts 3.13E-03 3.11E-03 Fetus, eyes 8.22E-03 8.45E-03

Ovaries 1.04E-02 6.75E-03 Fetus, spinal cord 7.52E-03 8.38E-03

Urinary bladder wall 1.88E-02 1.10E-02 Fetus, thyroid 6.65E-03 6.99E-03

Esophagus 4.31E-03 4.29E-03 Fetus, lungs 8.29E-03 9.34E-03

Liver 4.97E-03 6.36E-03 Fetus, thymus 7.48E-03 8.10E-03

Thyroid 2.63E-02 2.57E-02 Fetus, heart 7.69E-03 8.61E-03

Endosteal region 3.69E-03 3.29E-03 Fetus, liver 7.61E-03 8.51E-03

Salivary gland 1.44E-03 1.04E-02 Fetus, kidneys 7.78E-03 9.46E-03

Adrenal 6.26E-03 7.51E-03 Fetus, adrenals 7.58E-03 9.19E-03

Kidney 7.01E-03 1.02E-02 Fetus, spleen 8.26E-03 1.02E-02

Pancreas 7.93E-03 9.81E-03 Fetus, SI wall 8.04E-03 9.65E-03

SI-wall 7.05E-03 1.03E-02 Fetus, LI wall 7.79E-03 9.47E-03

Spleen 6.96E-03 7.85E-03 Fetus, bladder wall 7.83E-03 9.28E-03

Thymus 3.63E-03 3.36E-03 Fetus, stomach wall 7.76E-03 9.18E-03

Uterus 8.22E-03 8.31E-03 Fetus, gall bladder wall 7.37E-03 8.35E-03

Remainder tissues 5.69E-03 5.89E-03 Fetus, pancreas 7.97E-03 9.63E-03

Amniotic fluid 8.71E-03 8.70E-03 Fetus, soft tissue 8.36E-03 9.08E-03

Umbilical cord 8.19E-03 9.66E-03 Fetus, skin 8.73E-03 9.51E-03

a, bOrgan dose estimates using cumulated activities from Russell et al. and ICRP Publication 53, respectively. cDose to the fetusreported by Russell et al. [4].

E. Hoseinian-Azghadi et al.744

FUNDING

This study was supported by Vice President for Researchand Technology of Ferdowsi University of Mashhad. (Grantno. 20425, 1/3/2012).

ACKNOWLEGMENTS

The authors thank Mohhamad-Javad Mehrabifar, at QaemMRI Center, Masshad, for providing MR images required inthis study. The author’s thanks and appreciations also go toobstetricians, Nahid Shahabi and Elahe Ghabouli-Shahroudiwho have willingly providing aborted 5-month fetus for CTscan. The head of the radiology department at Imam-rezahospital, Masshad, is also appricated.

REFERENCES

1. ICRP (The International Commission on RadiologicalProtection). Pregnancy and medical radiation. ICRP Publication84. Oxford, UK: Pergamon Press, 1999.

2. Stabin M, Watson E, Cristy M et al. Mathematical models andspecific absorbed fractions of photon energy in the nonpreg-nant adult female and at the end of each trimester of pregnancy.Oak Ridge, TN: Oak Ridge National Laboratory, 1995.

3. Chen J. Mathematical models of the embryo and fetus for usein radiological protection. Health Phys 2004;86:285–95.

4. Russell JR, Stabin MG, Sparks RB et al. Radiation absorbeddose to the embryo/fetus from radiopharmaceuticals. HealthPhys 1997;73:756–69.

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APPENDICES

Appendix A: Segmentation of MR image sets

Contours segmented from MR images included the maternaloutline, exterior boundary of uterus, interior boundary of

Fig. 6. Comparison of dose maps calculated with two sets ofcumulated activities involving both photons and electroncontributions of Tc-99m.

Development of a 9-months pregnant hybrid phantom 745

uterus, exterior boundary of muscles, interior boundary ofmuscles, cervix, urinary bladder, large intestine (LI), smallintestine (SI), blood vessels, pelvis, spine, spinal cord,kidneys, spleen, liver, gall bladder, placenta and umbilicalcord. Moreover fetal outline, brain, liver, heart, lungs,stomach, SI, LI, urinary bladder, kidneys, gall bladder, eyes,spinal cord, mandible and pelvis were segmented from MRimages. Image contrast was limited in the fetal abdominalregion. Consequently, fetal adrenals, spleen, pancreas,thymus and thyroid were later added manually duringNURBS and polygon mesh modeling. Walls of the stomach,SI, LI, gall bladder and urinary bladder were later addedduring the voxelization process. The fetal skeleton was seg-mented from a previous CT scan of an aborted 5-month fetusand used in this model (Fig. A1).

Appendix B: NURBS and polygon mesh modelingand merging to ICRP reference phantom

OrientationIdeally, the segmentation process would occur within asingle image set, permitting correct orientation of theanatomy immediately upon being imported into anotherprogram. Because segmentation included three image sets(axial, coronal and sagittal), it was necessary to correctlyorient and align the three sets of polygon objects. Thosestructures were then oriented to the polygon mesh version of

the adult female ICRP reference voxel phantom (AF), aprocess guided by the skin contour and the locations of otherbones such as pelvis and spine (Fig. B1).

Organ replacement and NURBSsurface constructionAll organs of the abdominal and pelvic region of the basephantom were replaced with polygon mesh objects ofthe pregnant model (except for the pelvis, spine, adrenalsand ovaries). Then, the following polygon mesh objectswere converted to NURBS surfaces: maternal bodyoutline, exterior boundary of uterus, interior boundary ofuterus, exterior boundary of muscles, interior boundary ofmuscles, cervix, urinary bladder, urinary bladder contents,ureters, liver, stomach wall, stomach contents, gall bladderwall, pancreas, spleen, kidneys, LI, gall bladder andplacenta, in addition to the fetal brain, eyes and spinalcord. As an example, creating of the maternal body outlineand LI NURBS surfaces are illustrated in Fig. B1.Conversion to NURBS surfaces occurred through one of

two processes, depending on the geometry of the organ orstructure. In the first process, segmented contours exporteddirectly from 3D-Doctor as Drawing Exchange Files (*.dxf )were imported into Rhinoceros. These contours were usedlater as a curve network to construct NURBS surfaces. In thesecond process, Rhinoceros’ ‘polyline on mesh’ tool wasused to generate a network of curves around the polygon

Fig. A1. Segmentation and polygon mesh model of fetal skeleton.

E. Hoseinian-Azghadi et al.746

mesh object. Subsequent curve refinements should be per-formed using ‘rebuild’ and ‘curve edit’ tools to obtainsmooth and optimum curves. The ‘curve network’ tool wasthen used to create a NURBS surface around the frame pro-vided by that tool. In all cases, after conversion, the originalpolygon mesh objects were deleted from the phantom.

FetusThe fetal eyes were replaced by two NURBS spheres locatedexactly in their correct position. To account for the fetal adre-nals, spleen, pancreas, thymus and thyroid gland in thephantom, previously developed NURBS or polygon meshobjects of these organs by our research group were volumet-rically down‐scaled and fitted over the anatomically correctposition. A model of the fetal skeleton generated from aformer set of CT scans of an aborted 5-month fetus wasimported into Rhinoceros. Since the fetal pelvis and mandiblewere segmented from MR images, the skeleton bones wererotated and rescaled to orient with those anatomy landmarks.

Maternal breastDuring the pregnancy, certain organs, such as the breasts,will have increased masses. The RPI-P series of pregnanthybrid phantom published the volume of Breasts at 9-monthgestation [9]. This value was used in the current model as thereference volume. The breasts’ tissue from ICRP femalevoxel phantom was replaced with a scaled model fromMakehuman open source software to represent the referencevolume.

Maternal spine and soft tissuesThe polygon mesh object of the spine segmented from MRimages and exported from 3D-Doctor does not provide suffi-cient anatomical details. So, the Rhinoceros ‘flow alongcurve’ tool was used to align and orient the base phantompolygon mesh model of the spine guided by the location ofthe segmented spine (Fig. B1). This technique was used tomodify the spinal cord and vessels.

Fig. B1. (a) Orientation of segmented structures (red) to the polygon mesh version of the adult female ICRP reference voxel phantom(green and gray) guided by the location of bones. (b, c) Creation of NURBS surface using ‘curve network’ tool. (d) Before (left) and after(right) orienting and aligning AF spine (blue) to the segmented spine landmark (red).

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